The present invention relates to production of semiconductor devices, especially of MESFETs, in general, and more particularly to a method of fabricating sub-half-micron-size gates on semiconductor substrates.
There are already known various methods of fabricating semiconductor devices. Many of these methods employ sequential deposition of layers of various materials, followed by a subsequent removal of at least portions of at least some of such deposited layers. In many instances, such processes involve, during one stage or another, the application of a photoresist layer to the underlying substrate or layers or formations already present thereon, exposure of selected regions of the photoresist layer to light, removal of the exposed regions of the photoresist layer, and application of the next successive layer over the remainder of the photoresist layer and the regions uncovered by the removal of the exposed photoresist layer therefrom. However, experience has shown that there is a practical limit on the sizes (that is, on the dimensions) of the areas of the photoresist layer that can be protected from exposure to light by masking the same. This limit lies in the vicinity of one micron and, using current masking techniques, it is almost impossible to expose, or leave unexposed, areas of the photoresist layer where the size of such areas, that is, at least one dimension thereof as considered in the plane of the support surface of the substrate, is less than half a micron, with any degree of assurance that the desired area of the photoresist layer will actually be exposed, or prevented from being exposed, to light.
Yet, there is a pronounced desire in the field of producing semiconductor devices, such as MESFET's, to fabricate gate contacts (hereinafter referred to as gates) at least one dimension of which less than half a micron, such as in the region of 0.2 to 0.25 micron, at least at its region of contact with the underlying substrate since, as experience has shown, semiconductor devices equipped with such relatively small-sized gates are capable of operating at higher frequencies than similar semiconductor devices with larger-size gates, at very low noise levels. Since, as mentioned before, it is very difficult if not impossible to fabricate such small-area gates by resorting to the conventional masking techniques, they are being currently produced by electron beam lithography. However, this method requires relatively expensive equipment and, consequently, the semiconductor devices produced by this technique are quite expensive.